If you are in a restaurant, you can say "여기, 김치 좀 주세요! yugi, kimchi jom juseyo!" If you ask to someone who you know and she/he is order than you, you can say "김치를 좀 먹을 수 있을까요? kimch-leul mug-eul-su it-seul-gga-yo?". If you say to your friend who are same age with you, you can say "김치 좀 먹고 싶다. kimchi jom muk-go sip-da"